Wavelength tunable laser with dispersive element

ABSTRACT

A laser being tunable in wavelength includes a first reflecting unit and a second reflecting unit, both reflecting units being arranged to at least partially reflect an incident beam of electromagnetic radiation towards each other, an optical path of said beam of electromagnetic radiation within said cavity, which is defined in length by said first and second reflecting unit, a dispersive device, which is arranged, such that a portion of said optical path of said beam of electromagnetic radiation traverses through said dispersive device, wherein said dispersive device comprises a dispersive characteristic representing a functional dependence of an optical path length of said portion with respect to wavelength of said electromagnetic radiation, wherein said optical path length increases with an increasing wavelength of said electromagnetic radiation.

This is the National Stage of International Application No.PCT/EP02/09582, filed 28 Aug. 2002.

BACKGROUND OF THE INVENTION

The present invention relates to wavelength tunable lasers, particularlyto wavelength tunable lasers selecting resonance modes ofelectromagnetic radiation provided by an internal or external energysource.

Wavelength tunable lasers are playing an increasing role in the field ofoptical industry, particularly in the field of light generating oroptical measurement devices.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improvedwavelength tunable laser. The object is solved by the features accordingto the independent claims. Preferred embodiments are provided by thedependent claims.

According to the present invention a wavelength tunable laser isprovided comprising a first and a second cavity end mirror, both mirrorsdefining an optical path length of a beam of electromagnetic radiation,which is reflected by each mirror into a direction towards therespective other mirror. A cavity is defined in length by both mirrorsforms a series of resonance modes out of a radiation spectrum. Thewavelength of these modes depends on the optical path length within saidcavity.

A dispersive device is arranged within the optical path of said beam. Aportion of said path thus lies within said dispersive device. Thedispersive device may additionally comprise an Anti-Reflex coating.

The dispersive device comprises a dispersion characteristic, whichrepresents a functional dependence of a length of an optical pathportion within said device with respect to the wavelength of saidelectromagnetic radiation, wherein said optical path length increaseswith an increasing wavelength of said electromagnetic radiation.

The dispersive device serves as a compensator to compensate all or someof the following dispersive effects: discrete mode selection incavities, and/or dispersive characteristics of other elements in thecavity. The dispersive characteristics include the refractive index n,the first derivative of n with respect to wavelength lambda, dn/dλ, orthe 2^(nd) derivative d²n/(dλ)², or even higher derivatives.

According to preferred embodiments of the invention, there are at leasttwo aspects to implement a dispersive device having this functionalbehavior:

-   -   1. using dispersive material having a refractive index, which        increases with wavelength. This feature is also called anomalous        dispersion. E.g., Silicon in the wavelength range 300 nm–370 nm.        In general this behavior exists on the high energy side of an        absorption peak so there is also high loss in this wavelength        range.    -   2. using a reflective multi-layer structure, which by means of        Bragg-reflection and in combination with a sequence of suitable        layer thickness reflects an incident beam of electromagnetic        radiation having a smaller wavelength by a layer, that is at or        near the surface of said multi-layer structure, while a beam        having a larger wavelength is reflected by a deeper layer. The        optical path length of the electromagnetic radiation having a        larger wavelength thus attains a larger value. To accomplish        this, said layers are, e.g., provided with sequentially        decreasing layer thickness, or equivalently, with increasing        refractive indices. In this case normal dispersion material may        be applied. This structure may also be realized in planar        waveguides using etching technologies        -   Examples of materials utilized to form such layers are:        -   AlGaAs or AlGaInP epitactically grown on a GaAs substrate,            InGaAsP epitactically grown on a InP substrate, AlGaN            epitactically grown on a GaN substrate; semiconductor            material such as Si or Ge deposited e.g. in a thermal            evaporation step; semiconductor material structured as bulk            material such as Si, GaAs, InP; alternating layers of            dielectric materials such as SiO₂, TiO, Ta₂O₅, SiN; polymer            material like PMMA; combinations of metals and polymers. It            is to be understood, that the dispersive device is not            restricted to the material composition as provided in the            foregoing.        -   The first and second cavity end mirror can be of any            reflective structure. E.g., a metallic or dielectric mirror,            cleaved facet of a semiconductor chip with or without            additional coating.    -   3. using an external cavity setup that uses a dispersing        reflector that has a larger cavity length for larger wavelengths        in such a way, that all wavelengths fulfill the resonance        condition of the cavity.

In one aspect of the present invention the dispersion characteristic ofthe dispersive device within said cavity is designed, such that thecavity does not have discrete modes. Rather, it comprises a flathomogeneous transmission behavior in a certain wavelength range, i.e.all wavelengths within a certain wavelength range fulfill the resonancecondition of the cavity.

A cavity comprising such a dispersive device may be used, e.g., in modelocked lasers (pulse lasers) such that locking of a broader wavelengthrange is possible.

According to a further aspect of the present invention, the dispersioncharacteristic of the dispersive device is designed to compensatetotally or at least partially the dispersion characteristic of thecavity without said dispersive device, i.e. the sum of thecharacteristics of the other optical elements within the cavity. Thecompensation refers but is not limited to the refractive index n, thefirst derivative of n with respect to wavelength λ, dn/dλ, and 2^(nd)derivative d²n/(dλ)² or even higher order derivatives.

Therefore, the dispersion characteristic is substantially opposite to adispersion characteristic revealed by other optical components of thecavity. The dispersive device can have a length of the optical path ofsaid beam, which increases with wavelength at least within a limitedwavelength range. An absolute value of the optical path length of thedispersive device depends on the device extension, the actual path thebeam takes through said device and the actual radiation wavelengthconsidered.

Other optical components within the cavity such as the first and secondcavity end mirrors, lenses, windows, gaseous material, solid material,in particular semiconductor material, beam splitters, etc. generallycomprise a dispersion behavior, according to which an optical pathlength decreases with increasing wavelength of the electromagneticradiation.

In case the geometrical length of the path that the beam takes from thefirst cavity end mirror towards the second cavity end mirror is fixed,the features according to this embodiment of the present inventionresult in a constant optical path length of the complete cavityincluding the dispersive device as a function of wavelength.

In practice, when designing a cavity according to this embodiment of thepresent invention, it is advantageous first to determine a dispersioncharacteristic of the cavity without the dispersive device, and then toform a dispersive device by applying materials and a structure to saiddevice, which essentially displays the opposite dispersioncharacteristic with respect to the cavity. In one embodiment of thepresent invention, this may be accomplished by, e.g., a dispersiontailored photonic crystal.

According to a further embodiment of the present invention a wavelengthtunable cavity is advantageously provided with the dispersive device.For this purpose the geometrical distance between the first and secondcavity end mirror along the optical path of the beam, which maycorrespond to a linear or a redirected beam, can be varied in order toshift the series of resonance modes.

It is generally desired, to select one of these modes using a wavelengthtunable filter. Since a resonance mode shifts in wavelength as a resultof the cavity change, the wavelength filter is tuned in response to saidcavity change in order to keep the resonance mode within the filterwavelength range.

The dispersive device implemented within the optical path of the beam ofradiation compensates for the normal dispersion behavior of the otheroptical elements within the cavity. Otherwise, the normal dispersionbehavior of these elements would inevitably result in a mode hop, whenshifting the resonance modes towards larger or lower wavelengths.Therefore, the present invention advantageously extends the wavelengthrange available for performing a wavelength tuning without leading to amode hop.

In a further embodiment the wavelength tunable cavity is provided with alaser source comprising a laser medium, preferably of semiconductormaterial, a back facet, which forms a first cavity end mirror, and afront surface, through which a beam is emitted towards the second cavityend mirror. Preferably, the cavity comprises an internal and an externalcavity.

With respect to a wavelength tunable cavity, the present invention maybe implemented as either one of a Littmann cavity, a Littrow cavity, alinear cavity or a ring cavity, but is not restricted to such cavitiesgiven. Rather, any wavelength tunable cavity comprising optical elementshaving a dispersion characteristic, which limits the tunable wavelengthrange can advantageously be provided with a dispersive device accordingto the present invention. The invention is not restricted to lasers, animplementation of wavelength tunable cavities in the field offiber-interferometers is also possible.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention, when taken in conjunction with theaccompanying drawings.

FIG. 1 displays the basic concept of a cavity according to a firstaspect of the present invention with two cavity end mirrors and adispersive device, the cavity having a same mode number m for differentwavelengths (a), and a comparison of a simple Fabry-Perot cavity withoutand with a dispersive device illustrating a resulting transmissionspectrum (b),

FIG. 2 shows a gain chip with tilted facets for low internal reflectionsbeing incorporated into a cavity having a dispersive device.

FIG. 3 shows displays an embodiment with the dispersive device beingincorporated into the gain material.

FIG. 4 displays a wavelength tunable laser having an external linearcavity with a dispersive device according to the present invention,

FIG. 5 displays a wavelength tunable laser having an external Littmanncavity with a dispersive device mounted on the second cavity end mirror.

FIG. 6 displays an external cavity setup that fulfills the resonantcondition for all wavelengths that hit the curved reflector. The curvedreflector features a chirped grating on its surface that reflectsdifferent wavelengths λ₁, λ₂ in such a way that their direction isreversed.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

A first embodiment of the present invention is shown in FIG. 1 a. Acavity 1 is defined by a first cavity end mirror 10, a second cavity endmirror 20, both mirrors facing each other, such that a beam of anelectromagnetic radiation 100 traverses between them by means of directreflection. The electromagnetic radiation may, e.g., be in-coupledthrough a semitransparent first or second cavity end mirror 10, 20.

Cavity 1 comprises an optical path length defined by the fixed distancebetween the first and second cavity end mirrors 10, 20. It correspondsto a sum of the individual optical path length represented bycontributions from the first cavity end mirror 10, a gaseous medium 15and the second cavity end mirror 20.

Within the optical path, that the beam of electromagnetic radiation 100takes from mirror to mirror, a dispersive device 30 is placed having adispersion characteristic, which is opposite to the dispersioncharacteristic of the first and second cavity end mirrors 10, 20 and thegaseous medium 15. I.e., the optical path length of a portion 31 of theoptical path within said dispersive device 30 increases with increasingwavelength. The dispersive device 30 also contributes to the totaloptical path length, which depends on the wavelength according to thedispersion characteristic.

Dispersive device 30 comprises a reflective multi-layer structuremounted on the second cavity end mirror 20, which also has an AR-coating35. It comprises a specific dispersion characteristic, the effect ofwhich is shown in the two resonance mode curves displayed in the bottomsection of FIG. 1 a. The upper curve corresponds to a first vacuumwavelength, or spectral energy E₁ respectively, while the bottom curvecorresponds to a second spectral energy E₂, whereby the first spectralenergy is larger than the second spectral energy. However, both curveshave the same mode number (m=10). A decrease in vacuum wavelength andthus a step towards a lower mode is compensated by a decrease in opticalpath length due to the dispersive device 30. As a result a slightlyincreased spectral energy is still represented by the same mode number.

For this embodiment a desired dispersion curve that has to beaccomplished by the dispersive device 30 can be calculated fromequations:m·λ ₁ =I _(vac) +I _(comp) ·n(λ₁)m·λ ₂ =I _(vac) +I _(comp) ·n(λ₂),where I_(vac) and I_(comp) are the geometrical path lengths throughvacuum and the dispersive device 30, respectively. From the conditionλ₁<λ₂, it follows that n(λ₁)<n(λ₂) and thus the optical path lengthwithin the dispersive device 30 at a wavelength λ₁ is smaller than at awavelength λ₂.

A comparison of a simple Fabry-Perot cavity without and with adispersive device illustrating a resulting transmission T as a functionof wavelength λ is shown in FIG. 1 b. The upper diagram displays theprior art status, where a set of resonance modes develops within thecavity. However, in the lower diagram representing the case including adispersive device 30 all wavelengths λ within a certain wavelength rangefulfill the resonance condition, which yields a flat transmissionspectrum. The flat spectrum of transmission T in this examplecorresponds to just one mode (m=10).

A cavity comprising a gain medium or an amplifier is shown in FIG. 2.Here, the dispersive device 30 is used with advantage in a ripple freeASE-source (ASE=amplified spontaneous emission), which is realized byinclined facets of the gain chip 50 with respect to the light beam 100.

A cavity having a dispersive device 30 for mode compensation integratedwithin a gain chip is displayed in FIG. 3. The integrated usage of adispersive device 30 becomes particularly advantageous in combinationwith a linear optical amplifier (LSOA), when a flat transmissionspectrum is to be generated by means of the cavity.

A further embodiment of a cavity according to the present invention isshown in FIG. 4. Cavity end mirrors 10 and 20 define an optical pathhaving a length within a linear cavity laser source. A gain medium 50having a front surface 51 and a back facet, which is identical to thefirst cavity end mirror 10, emits a beam 100 of electromagneticradiation through a lens 40 towards the second cavity end mirror 20.Lens 40 serves for collimating the light beam emitted from the lasersource along said optical path.

The cavity 1 additionally comprises a wavelength tunable filter 60. Itis connected with the movable second cavity end mirror 20 via a controlunit 80. A shift of the movable second cavity end mirror 20 results in awavelength shift a multiple of resonance modes within cavity 1. By meansof control unit 80, said wavelength tunable filter 60 is adapted in itsfilter wavelength range, such that this range co-moves with a desiredresonance mode.

Some optical elements of the cavity, e.g., the gain medium 50, which isa semiconductor chip, the lens 40, the wavelength tuning filter 60, etc.display a dispersion characteristic 110, which can be seen in the bottomleft section of FIG. 4. In the schematic representation of a diagram therefractive index n is given as a monotonically decreasing function ofwavelength λ.

A dispersion device 30 is supplied within cavity 1 that has a dispersioncharacteristic 120 displayed in the bottom right section of FIG. 4.Here, the refractive index n is a monotonically increasing function ofwavelength λ. The dispersion characteristic 120 of said dispersiondevice 30 is designed to compensate the dispersion characteristic 110 ofsaid other optical elements 10, 20, 40, 50, 60 within said cavity 1.Weighting the refractive index with a geometrical length of said devicealong said optical path of said beam determines the actual optical pathlength, respectively.

Accordingly, using the cavity 1 of this embodiment, a difference inoptical path length due to optical elements such as the lens 40, thelaser medium 50, the wavelength tunable filter 60, etc. when scanningthrough a resonance mode wavelength by means of actuating the secondcavity end mirror can be outweighed by a corresponding negativedifference in optical path length due to a dispersive device 30 havingan appropriate dispersion characteristic.

A third embodiment of the present invention is shown in FIG. 5, where aLittmann cavity is displayed. A first and second cavity end mirrordefine a length of an optical path of a cavity, in which an inclinedgrating is placed as a tunable wavelength filter. The second cavity endmirror is movable in a direction of rotation 200 about a pivot point 91,which is defined as an intersection between a line drawn as an extensionfrom the grating 60, the mirror 20 and the back facet, which isidentical to the first cavity end mirror 10. The cavity also comprises alaser medium 50 and a lens 40 for collimating a beam 100 emitted fromsaid laser medium 50.

A dispersive device 30 is mounted as a reflective multi-layer on thesecond cavity end mirror. As in the previous embodiment its dispersioncharacteristic 120 is designed to compensate the dispersioncharacteristic 110 of other optical elements. Advantageously, byrealizing a dispersion-free cavity a pivot point 91, which has a stableposition within a wavelength range of larger than 400 nm, becomespossible as compared to about 150 nm wavelength ranges of dispersionlimited systems.

In this third embodiment the dispersion compensating device may beplaced at any part of the optical beam 100.

In a further embodiment a dispersion device 30 has a controllabledispersion characteristic, e.g., by means of mechanical pressure orelectrically by means of a piezo-element. For example, the thickness ofsingle layers of a reflective multi-layer can be influenced in order toattain a desired dispersion characteristic.

A still further embodiment of an external cavity setup that uses adispersing reflector, which has a larger cavity length for largerwavelengths in such a way, that all wavelengths fulfill the resonancecondition of the cavity, is given by the arrangement shown in FIG. 6.The configuration is of a Littmann-type. There, a dispersive device 30is realized by a chirped grating. It reflects the light beam 100redirected by the wavelength filter 60, which is also represented by agrating.

The effect of the grating 60 is that portions of the light beam havingdiffering wavelength are redirected, i.e. diffracted, under differentangles towards the dispersive device 30. The grating of the dispersivedevice 30 is chirped or curved, such that it forms a surface of a sphere90 having a radius r, which is half the distance between the pivot point91 of the cavity and the intersection of the light beam 100 with thefilter grating 60, wherein the midpoint M of the sphere 90 is positionedat the centre on a line connecting the filter grating 60 and the pivotpoint 91. By this configuration portions of the diffracted light havinglarger wavelengths λ₂ are diffracted with a larger angle than portionshaving smaller wavelengths λ₁. As a result the path between the filtergrating 60 and the dispersive device grating 38 is longer for largerwavelengths. The dispersive device 30 in this embodiment consists of thechirped grating 39 and the space 38 between both gratings.

While the invention has been shown and is worked out in detail, theforegoing description is in all aspects illustrative and notrestrictive. It is therefore understood, that numerous modifications andvariations can be devised without departing from the scope of theinvention.

1. A laser being tunable in wavelength, comprising: a first reflectingunit and a second reflecting unit, both reflecting units being arrangedto at least partially reflect an incident beam of electromagneticradiation towards each other, an optical path of said beam ofelectromagnetic radiation within a cavity defined in length by saidfirst and second reflecting units, a dispersive device arranged suchthat a portion of said optical path of said beam of electromagneticradiation traverses through said dispersive device, wherein saiddispersive device comprises a dispersive characteristic representing afunctional dependence of an optical path length of said portion withrespect to wavelength of said electromagnetic radiation, wherein saidoptical path length increases with an increasing wavelength of saidelectromagnetic radiation, and a gain medium for generating saidelectromagnetic radiation, said gain medium comprising a back facet,which is identical to said first reflecting unit, and a front surface,said gain medium emitting said beam through said front surface towardssaid second reflecting unit.
 2. The laser being tunable in wavelengthaccording to claim 1, wherein either one of said gain medium or saidsecond reflecting unit is movable in the direction of the optical pathof said beam for adjusting said optical path length of said cavity tosaid selected wavelength range provided by said wavelength tunablefilter.
 3. The laser being tunable in wavelength according to claim 1,comprising at least one of the features: said gain medium is a linearsource optical amplifier; said dispersive device is integrated withinsaid gain medium.
 4. A laser being tunable in wavelength, comprising: afirst reflecting unit and a second reflecting unit, both reflectingunits being arranged to at least partially reflect an incident beam ofelectromagnetic radiation towards each other, an optical path of saidbeam of electromagnetic radiation within a cavity defined in length bysaid first and second reflecting units, a dispersive device arrangedsuch that a portion of said optical path of said beam of electromagneticradiation traverses through said dispersive device, wherein saiddispersive device comprises a dispersive characteristic representing afunctional dependence of an optical path length of said portion withrespect to wavelength of said electromagnetic radiation, wherein saidoptical path length increases with an increasing wavelength of saidelectromagnetic radiation, and a wavelength tunable filter for selectinga wavelength range of a spectral distribution of said electromagneticradiation comprising one resonance mode out of the set of resonancemodes of said cavity.
 5. The laser being tunable in wavelength accordingto claim 4, wherein said functional dependence of said dispersivecharacteristic is designed to admit exactly one single mode ofelectromagnetic radiation to develop within said cavity.
 6. The laserbeing tunable in wavelength according to claim 4, wherein saidfunctional dependence of said dispersive characteristic is designed suchthat said optical path length within said cavity is the same for any twodifferent wavelengths of said electromagnetic radiation at least withina limited wavelength range.
 7. The laser being tunable in wavelengthaccording to claim 4, wherein said dispersive device includes at least apart of said second reflecting unit.
 8. The laser being tunable inwavelength according to claim 4, further comprising a lens forcollimating said beam emitted from said gain medium along said opticalpath towards said second reflecting unit.
 9. The laser being tunable inwavelength according to claim 8, wherein said dispersive device includesat least a part of said lens.
 10. The laser being tunable in wavelengthaccording to claim 4, wherein said wavelength tunable filter comprises agrating for diffracting and redirecting said beam of electromagneticradiation, the cavity being either one of a Littrow cavity or a Littmanncavity.
 11. The laser being tunable in wavelength according to claim 4,wherein said dispersive device comprises one or more materials of thegroup comprising: semiconductor material epitactically grown on asubstrate material, said semiconductor material and said substratematerial being either combination of: AlGaAs and GaAs, AlGaInP and GaAs,InGaAsP and InP, or AlGaN and GaN, respectively, a semiconductormaterial deposited on a substrate material in a vapor deposition step,said semiconductor material being one of a group comprising: Si, Ge, asemiconductor material structured as bulk material being one of Si,GaAs, and InP, a dielectric material being of SiO₂, TiO, Ta₂O₅, SiN, apolymer material of a group comprising PMMA.